This invention relates to semiconductor devices and, more particularly, to batch processing semiconductor lasers.
In most previous work, the procedures used to fabricate discrete heterostructure lasers from Group III-V compounds and alloys have included mirror formation by breaking the crystal along (110) cleavage planes. Many of the lasers thus fabricated have excellent properties, but there are many disadvantages inherent in the fabrication of laser mirrors by cleaving. Foremost among these disadvantages is the loss of the ability to batch process the devices after die separation (i.e., cleaving). Rather, each die has to be individually handled for subsequent processing (e.g., applying facet coatings) and for testing. Cleaving also places restrictions on laser geometry, including minimum laser length. Although shorter lasers have a number of distinct advantages, including lower threshold current (lower power consumption) and wider longitudinal mode spacing (single mode operation), the fabrication of such lasers by conventional cleaving techniques is difficult to perform reproducibly and has a low associated yield. In contrast, a discrete laser batch fabrication process, without the geometrical restrictions implicit in the cleaved mirror approach, should open the way to the realization of a number of advanced electro-optical components. By way of example, such components might include a semiconductor laser fabricated on the same chip with a photodiode for feedback control or with a transistor for drive control.
In the past, however, potential integrated optics applications rather than batch fabrication processing has prompted workers to seek alternatives to cleaving the laser mirrors. The dominant alternative seems to be chemical etching. A decade ago, A. S. Dobkin et al [Sov. Phy. Semicond., Vol. 4, page 515 (1970)] demonstrated the etched mirrors could be fabricated on (100)-oriented GaAs homojunction lasers by etching with a peroxide-alkali solution (H.sub.2 O.sub.2 and NaOH), and predicted that such lasers had application in the field of integrated optics. Later, C. E. Hurwitz et al [Appl. Phys. Lett., Vol. 27, No. 4, page 241 (1975)] brought that prediction closer to reality by using an acidic-peroxide solution (1H.sub.2 SO.sub.4 :8H.sub.2 O.sub.2 :1H.sub.2 O) to etch mesas into (100)-oriented GaAs-AlGaAs double-heterostructure (DH) wafers. Opposite, parallel mesa walls were formed perpendicular to the (100) surface and defined the Fabry-Perot cavity resonator. Radiation at 9100 Angstroms was coupled through the mirrors into thick GaAs waveguide layers grown adjacent to the mesa. A significant advance was then made by R. A. Logan et al [U.S. Pat. No. 4,136,928 (1979)] who recognized the utility of making the AlGaAs DH laser mirrors oblique to the resonator axis (rather than perpendicular to it) in order to couple the laser radiation into an underlying waveguide. The mirrors were (111) crystal facets and were formed by orienting the lasers along the &lt;110&gt; direction and exposing the active layer to a superoxyl etchant (H.sub.2 O.sub.2 and H.sub.2 O, pH .about.7).
Although Dobkin et al do not specify how their lasers were masked for the etching step, Hurwitz et al used a photolithographically defined pyrolytic SiO.sub.2 mask to define rectangular mesas. As depicted in FIG. 1, a sputtered Cr-Au contact was made coextensive with the top of the mesa. In contrast, Logan et al (column 5, lines 1 et seq.) used standard photolithography to form rectangular resist patterns on a previously evaporated Au contact layer. Subsequent etching undercut the contact layers 24 and 26 as shown in the drawing.